Paul-
"Though I do find it difficult to believe that we cannot get a lot better millage from cars today. After all I got way over 50 MPG in my 1980 VW Diesel and it is more than 30 years later! "
There ARE TDI engines that will getbetter mileage and poewr than your non-turbo-diesel got. The problem is, they are illegal on the US marlet because almost all diesels spew particulates and the EPA is convinced those are carcinogens that also cause allergy and respiratory problems. And it looks like the EPA is right on that one, more evidence keeps saying the same thing.
The EU has allowed them but starting "now" you will see all sorts of new diesels coming to market, most with an extra tank of urea which is injected into the exhaust in order to clean it up. I'd guess that will impact the boat market soon too.